Selective transmission of electric waves



Patentes .hay is, ieee.

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HARLD E. VEN NES. 0F WESTMGUNT, QUEBEC, CANADA, ASSIGNGR T0 WESTERNELEG- TRIO CCMIPANY, INCORPORATED, OF NEW vYORK, N. Y., A CORPORATION FN'EW YORK.

SELECTIVE TRANSMSSION GIF ELECTRIC I Application filed July 26, 1922.

residing at Westmount, in the Province or 5 Quebec, Dominion of Canada,have invented certain new and useful In'iprovements in the SelectiveTransmission of Electric `Waves, of which the following is a `full,clear, conoise, and exact description.

This invention relates to the selective transmission of electric wavesand particularly to the prevention of interference due to stray orleakage currents in suc-h a system.

This invention provides an improved arrangement or associating electricwave or alternating currenttransmitting and receiving circuits with atransmission medium or line. In a more limited aspect it provides animproved arrangement for associating carrier wave transmitting andreceiving circuits With a transmission medium or line.

For a general description ot carrier systems, reference may be had .to apaper entitled Carrier current telephony and telegraphy, byvMessrs'.Colpitts and Blackwell, published in the Journal of the AmericanInstitute of Electrical Engineers, for April, May and June, 1921. In thesystems therein described, the carrier terminal apparatus is connectedto the carrier line through a balanced transformer, so that thetransmitting and receiving circuits are conjugately connected to theline with respect to one another, a balancing network being used tobalance the impedance of the line. A more simple arrangement consists indirectly connecting the transmitting and receiving circuits to thetransmission line, and preferably to employ distinctive 'frequencies fortransmissions in opposite directions. Such an arrangement, however, isoften trouble some due to stray or leakage currents arising from theunbalanced arrangement of the terminal circuits.

An object of this invention is to provide a simple terminal arrangementfor electric wave transmission systems, which is free from noise andinterference due tostray or leakage currents in the system, but. whichstill makesuse of a simple connection between the terminal apparatus andthe transmission medium or line. Another object is to provide such asimple arrangement for Serial N9. 5??,493.

carrier current signaling systems. Other objects of the invention willbecome apparent as the description proceeds.

The term transmission medium as used in tht; specification is vto beunderstood to embrace a transmission circuit including both metallicconductors and other media for the transfer of electrical energj7between two points. The electrical midpoint of such medium or ot' atransmission line refers to the positions at which the potential ismidway between the desired driving potentials. For example, if a.certain driving otential is applied between the line wires o atransmission line, the electrical midpoint of the line would be at thepoint where the p0- tential drop from one wire to said midpomt is equalto the potential drop from said midpoint to the other wire.

In the preferred embodiment of the in vention, a shielded transformer isconnected between a common carrier receiving circuit and a carriertransmission line, while a common transmitting ciicuit is connected tothe electrical, midpoint ot' the line side of the transforn'ier. Aplurality7 of carrier receiving branches are connected to the commonreceiving circuit on the one hand, and a plurality ot' carriertransmitting branches, to the transmitting circuit on the other. Anelectrostatic shield which is maintained at ground potential is locatedbetween the line winding and the station Winding of the transformer. Theinvention, however, is applicable to a terminal station comprising onlyone transmitting and one receiving branch.

Referring now to the drawing which shows the preferred embodiment of theinvention, a carrier transmission line ML is connected to a carriertransmitting circuit TC, and a carrier receiving circuit RC b means of atwo winding transformer 5.

high pass filter HPF is inserted between'the "transformer 5 and the lineML to allow the passage of carrier currents, but to substantiallyprevent the passage of' currents having essential frequencies inthevoice range or below. This filter is of the'generel ty e described inthe patent to GeA. Campbell, No. 1,227,113, May 22, 1917.

An ordinary telephcneli-ne TL may also be connected to the transmissionline ML'.

Q i,cci

low pces lter LPF of the general type 'described in the ahove mentionedCampbell patent is inserted in the line TL to allow the passage ofcurrents of essential voice frequencies and lower frequencies and 'toprevent the liow of carrier currents to the line TL. A

A transmitting branch comprising e source O of carrier frequency Waves,modulator M, and a transmitting band filter SBF,

is connected to the transmitting circuit TC.

A receiving brunch comprising e .receiving band filter BBF, deinoduletorl), deniodula-v tor amplifier A., and a low pass voice fre quency filterVF, is connected to the receiv ing circuit RG. 'A low frequency line Lis` associated with the transmitting and receiving branches by ineens ofa balanced treinsformer 6. A low frequency network LN balances the lineL, so that the transmitting and receiving channels are conjugateiyconnected toy the line L with respect to one another.

The oscillator O, modulator M, detector D, and detector amplifier A, erepreferably of the well lniowii vacuum tube types, 'out any othersuitable type of apparatus may he substituted.' therefor. The band ltcrsSBF and BBF are of the generul type described in the Cumplxill patenthereinbefore :inen-v tioned, differing from euch other only in theconstants of the inductence and capacity elements, the velues of whichdepend upon the bend of frequencies which it is desired to transmit.

Other transmitting und receiving branches I may be connected to thetransmitting circuit TC, and to the receiving circuit RFC, as indicatedby blocks 7, 8 and 9, l() respectively. In the preferred system, eachbend iilter passes a distinctivebund of frequencies, and

the transmitting frequencies as e. group occi'ipy a diiferent range fromthe receiving frequencies as a group. For exemple at a giventerniinalstation carrier frequencies of 6, 9 and 12 kilocycles might be used forAtransmitting and carrier frequencies of l5,

y 18 and 21 kilocy'cles, for receiving.

At the distant station the group of higher frequencieswould be used fortransmitting and' the group-of lower frequencies, 'for receiving. Theend sections of these band filters, adjacent to the transmission line MLare of low impedance to all frequencies, except those which theindividua-l tilteris designed to pass to' or from the line ML as thecese maybe. These end sections of all ofthe transmitting filters areconnected iii series with each other and with the transmitting circuitTC. Likewise the end sectionsof all of the receiving band filters areconnected in series with each other and with the receivcircuit RC.

the preferred system, a grouping filter GF is inserted in the receivingcircuit RC,

exclude the range of frequencies used for transmission. Likewise, agrouping filter TGF (shown in dotted lines, since it is not actuallyused inf the preferred system) may be inserted in the transmittingcircuit TC to freely transmit the frequencies employed for'transmitting, and to substantially exclude the frequencies employed forreception at this station. vBotlrthe filter GF and the filter TGF maybeomitted, if the band filters in the several transmitting and receivingbranches provide satisfactory discrimination between the transmittingand receiving frequencies. These filters are likewise of the generaltype described in the aforementioned Campbell patent. If the groupingfilters GF and TGF are both omitted, it is obvious .that the groupedarrangement of transmit ting 'anclreceiving frequencies need not beused. The so-callcd staggered frequency arrangement may be foundpreferable.

`,The transformer 5, has been described as a two winding transformer. Bythis is meent that in respect to its transforming ac tion, it .may betreated as having only two windings, one called a station winding towhich the receiving circuit RC is connected, end the other a linewinding to which the transmission line MIL is connected. In fact,however, tholinc wind n t leest is composed of two electrically i nticalwindi. so 4that the transmitting. circuit TC may 'connected in seriestherewith, undgto the eleci ground potential'. This grounded shieldfurnishes a short circuit path to ground for any unbalance currentsoriginating in the line ML or any other of the circuits connected to theline winding of transformer 5. Such unbalance currents would otherwisefind their way into the receiving circuit RC, due to the capacitybetween the line and station windings, and would cause interference andnoise,

In a complete carrier system another station similar to theoriedescribed and shown in, the drawing, would be provided at the distantend of the transmission line ML. This station would be so arranged thatthe transmitting branches'at the station de, scribed, would cooperatewith lthe branches at the Adistant station, and t transmitting branchesat the distant station would coo crate with the receiving branches atthe station shown. vThe meth tion will not be described, since it isohvious from the Colpitts and Blackwell paper hereinbefore mentioned.

It should be noted that since the end section of. the grouping filterGF, if such a filter is used, or the end sections of the receiving bandfilters if iro grouping lter GF is employed, vare of low impedance tothe transmitted frequencies, and therefore actas a short circuit for thestation 4Winding of the transformerv 5, and consequently the transformer5 o'ers small impedance to the transmitted frequencies. This followsfrom Well known transformer theory. The self inductance of the primaryor line Winding is largely neutralized by the magnetic effect of theshort circuit current in the secondary or station Winding, so that theprimary impedance is greatly reduced over what it would he with an opencircuited secondary. Also, the end section of the transmitting groupingiilter TGF or the end sections of the transmitting hand filters, if thefilter TGF is not employed, are of low impedance to the received'frequencies, and therefore function as a direct low impedance connectionbetween the electrical midpoint terminals of the line Winding for thesefrequencies. Thus a balanced terminal circuit isobtained `with verylittle decrease in transmission eiliciency, over what would he'ohtainedwere the transmitting and receiving circuits directly connected to thetransmission liiiell/[L Thev invention has heen described in itsembodiment in a multiplex carrier telephone system, hut its use is notlimited to that speciiic system.

What is claimed is:

1.' An alternating current system cornprising a transmission line, atransmitting circuit for impressing alternating current upon said line,another circuit for receiving alternating current from said line, and atransformer having a line. lWinding and a station Winding, saidreceiving circuit being connected to said station Winding, said lineWinding being connected to said line, and said transmittingcircuit beingconnect ed in series with said line Winding at its electrical midpcnt.-

2. A carrier terminal system comprising a transmitting circuit, areceiving circuit, a ,transmission line, and a two winding transformerhaving one of its windings connected to the receiving circuit, the otherconnect ed to the transmission vline and the transmitting circuitconnected in series with the line Windingat its electrical midpoint.

3. A carrier terminal system comprising a transmittin ',circuit, areceiving circuit, a transmission ine, a source of lcarrier current ofoperaot characteristic frequency for said trans`- mltting circuit, aselect1ve circuit adapted to pass this characteristic frequencyconnected between said source of carrier current andv said transmittingcircuit, a detector for carrier current of a different frequency, a se.lective circuit adapted to pass current of said different frequencyconnected between said receiving circuit and said detector, andatworwinding transformer having one of its windings connected to thereceiving cir-` transformer having a line winding and av stationWinding, said receiving circuit -.be-

ing connected to said station Winding, said;l

line Winding being connected to said trans-` mission line and saidtransmitting circuit bei connected in series with said line win ing atits electrical midpoint.

5. ln a multiplex carrier telephone terminal system, a transmissionline, a trans!- mitting circuit, a plurality of transmitting brancheseach comprising means to modulate a characteristic carrier frequencyWave 1n accordance with voice frequencies and a hand filter adapted totransmit Waves of the resulting hand of frequencies, means to connectthe end sections of said hand filters in series with each other and withsaid transmitting circuit, a receiving circuit, a pluraiity of receivingbranches each compris ing meanseto demodulate Waves of a characteristichand of frequencies and a band filter adapted to transmit Waves of saidcharacteristic cand'cfv frequencies, means to connect the end sectionsof said last named band filters in series with each other and said re'ceiving circuit, and a transformer having a line winding and a stat-ionWinding, said receiving circuit being connected to said station Winding,said'line Winding bein connected to said transmission line, an saidtransmitting circuit 'heilig connected in series with said line windingat its electrical midpoint.

6. ln a multiplex carrier terminal system, a transmission line, atransmitting circuit, a

plurality of transmitting branches cach coinprising a hand filteradapted to pass a characteristic handof frequencies and hav ing an endsection of low impedance to all frequencies except those which it isadapted to transmit, means to connect said and sec,u tions in serieswith each other and with the transmitting circuit, a receiving circuit,a plurality of receiving branches each 'c0m llo of said. last namedfilters in series with each other and with the receiving circuit, and. stransformer having e line winding and a station winding, seid receivingcircuit being connected to seid station winding, seid line Winclingybeing connected to seid transmission. line and said transmitting circuitbeing connected in Ser-ies with said line Winding at its electrica-lmidpoint.

7. In e multiplex carrier terminal system, a transmission line, etransmitting circuit, a plurzility of transmitting branches eachcomprising-a band filter adapted to ess a characteristic hand offrequenciesen hsving an end section of low impedance to all frequenciesexcept thosevvhich it Ais adapted to transmit, means to connect said endsections in series with each other and with the inerte-i transmittingcircuit, ,a receiving circuit, e

plurality oit receiving branches .a hcom- 25. i prising a band lteradapted to pets. a cliery acteristie band oit frequencies and having vend sections of low imedence .to .all reqnencies exceptv those which itis adapted to transmit, means to connect,4 said lest recited `30 endsections in series with each other end with the receivin circuit, atransformer' having e line Wining and e station Wintiing, said receivingcircuit being connected to seid ste-tion Winding., said linewinding 35being connected to said transmission line and seid transmitting circuitbeing con` nected` in series 4with said line Windinget its electricalmidpoint, and an electrostatic shield between the windings of saidtrans- 40 former, said shield being maintained at! ground potentiai Y 1nWitness whereof, I hereunto subscribe my neme this 18th dey of July A.D, 1922.

l HARALD J. VENNES.

